Hess’s Law Calculator
Combines known reaction enthalpies into the one you want. Work out from the equations how each step has to be scaled so that they add up to the target.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
ΔH = Σ (multiplier × ΔH of the step)
Enthalpy depends only on the start and end, so the route does not matter. Reversing a step flips its sign, and doubling it doubles the enthalpy.
What to watch out for
The default values make C + ½O₂ → CO: the combustion of carbon, minus the combustion of carbon monoxide.
Frequently Asked Questions
What is the enthalpy of forming carbon monoxide from these steps?
−393.5 − (−283.0) = -110.5 kJ/mol.
How do I use enthalpies of formation?
Enter each substance as a step, with its coefficient as the multiplier for products and minus its coefficient for reactants. That gives ΣΔHf(products) − ΣΔHf(reactants).